The present invention relates to a sheet feeding device for an image forming apparatus and more particularly to a sheet feeding device built in or operatively connected to a printer, copier or similar image forming apparatus.
A stencil printer belonging to a family of printers includes a print drum around which a master is wrapped. A press roller, press drum or similar pressing means presses a sheet fed from a sheet feeding device at a preselected timing against the master. As a result, ink is transferred from the inside of the print drum to the sheet via the perforations of the master, forming an ink image on the sheet. In a copier, for example, a toner image is transferred from an image carrier to a sheet fed from a sheet feeding device at a preselected timing.
The sheet feeding device is built in or operatively connected to the image forming apparatus and includes a tray or a cassette loaded with a stack of sheets. A pickup roller contacts the top sheet and pays it out. A separator pad or separating member and a separator roller cooperate to separate the top sheet being paid out by the pickup roller from the underlying sheets. This kind of sheet separation, generally referred to as a friction separation system, causes a greater frictional force to act between the separator pad and the sheets than between the sheets.
A stencil printer, among others, is operated with various kinds of sheets. Sheets are generally classified into standard sheets, thin sheets and thick sheets or more minutely into standard sheets, rough sheets, thin sheets, thick sheets, and special sheets. As for the minute classification, standard sheets include high quality sheets (high quality 55 kg sheets, high quality sheets for stencil printers and so forth), medium quality sheets, and recycled sheets. Thin sheets include thin Noshigami (a piece of paper customarily attached to a gift in Japan) and high quality 45 kg sheets. Thick sheets include high quality 135 kg sheets or above, drawing paper, and postcards. Special sheets include rectangular envelopes.
Sheets of each kind or each size have particular quality including thickness and surface condition and a particular weight. Therefore, the frictional force depends on the kind and size of sheets and sometimes renders sheet separating conditions in adequate. This is apt to cause a plurality of sheets to be fed at the same time (over lap feed hereinafter) or cause no sheets to be fed (feed failure hereinafter) or cause thick sheets to peel off, proving that the optimal sheet feed conditions including a feed pressure and a separation pressure depend on the kind and size of sheets.
As for a stencil printer operable with various kinds of sheets, as stated above, it is difficult to optimize sheet feed conditions for all kinds of sheets by simply adjusting the feed pressure, separation pressure and so forth stepwise with a single separator pad or a single pad angle.
Generally, the sheet feed conditions become inadequate and bring about defective sheet feed, depending also on temperature, humidity and other environmental conditions. For example, when temperature or humidity drops, the overlap feed is apt to occur. In light of this, a high separation pressure and a low feed pressure are selected. When temperature or humidity high, a low separation pressure and a high feed pressure are selected because the feed failure is apt to occur.
On the other hand, for a given separation pressure, the frictional force to act and therefore the sheet feeding ability depends on the material and surface condition of the separator pad. It is therefore a common practice to classify sheets by kind and size and prepare a plurality of different separator pads each matching with a particular class of sheets as determined by experiments. An optimal separator pad is selected in accordance with the kind and size of sheets to be used.
However, in most of conventional sheet feeding devices, the materials of the separator pads and pad angles are fixed and cannot be switched. As a result, when the sheet feeding device is frequently operated, i.e., when a great number of sheets are fed, the separator pads must be frequently replaced due to wear. The kind of the separator pad and pad angle, if switchable, are switched by hand. Manual switching operation is not easy and is therefore extremely troublesome to perform.
The above problems with the conventional sheet feeding devices may be summarized, as follows.
(1) The kind of the separator pad and pad angle which cannot be automatically switched are troublesome to replace. It is therefore impractical to set up optimal sheet feed conditions matching with the kind of sheets to be used or temperature, humidity and other environmental conditions, resulting in jams, overlap feed, peeling and other defective sheet feed.
(2) The kind of the separator pad and pad angle, if switchable, cannot be easily switched. This, coupled with the fact that the switching operation relies on the operator""s experiences, makes it difficult to select optimal sheet feed conditions. Further, although the kind of the separator pad and pad angle may be variable in accordance with the kind and size of sheets, the operation for varying the sheet feed conditions is extremely troublesome and delicate to perform. As a result, printing, for example, is often executed without the optimization of the sheet feed conditions, again resulting in defective sheet feed. This prevents merits achievable with the switching of the kind of the separator pad and pad angle from being made most of.
(3) When spare pads are not available at the time for replacing the separator pad in use, a long period of time is necessary for replacement, or the apparatus is killed over a long period of time to simply wait for the delivery of spare pads.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 5-229243, 930714, 9-235033, 7-125855, 8-108947, 8-301500, 9-86692, 9-208058 and 10-139191, U.S. Pat. No. 5,927,703, and U.S. patent application Ser. Nos. 08/925,648, 09/222,820, and 09/135,856.
It is therefore an object of the present invention to provide a sheet feeding device for an image forming apparatus capable of automatically switching the kind of a separator pad and/or a pad angle without any troublesome manual operation.
It is another object of the present invention to provide a sheet feeding device for an image forming apparatus capable of automatically switching a separating member and/or the angle of the separating member in accordance with the kind of sheets to be used and temperature, humidity and other environmental conditions.
It is still another object of the present invention to provide a sheet feeding device for an image forming apparatus capable of automatically selecting and setting optimal sheet feed conditions matching with the kind of a separating member and/or the angle of the separating member automatically selected, thereby guaranteeing optimal sheet feed conditions at all times.
It is a further object of the present invention to provide a sheet feeding device for an image forming apparatus obviating an occurrence that a long period of time is wasted for replacement due to the absence of spare pads or that the apparatus is killed over a long period of time due to the absence of spare pads.
In accordance with the present invention, a sheet feeding device for an image forming apparatus includes a plurality of separating members each having a particular coefficient of friction with respect to a sheet for separating sheets one by one, and an automatic separating member switching mechanism for automatically selecting one of the separating members.
Also, in accordance with the present invention, a sheet feeding device for an image forming apparatus includes a plurality of separating members for separating sheets one by one, and an automatic separation angle switching mechanism for automatically switching an angle of the separating members.
Further, in accordance with the present invention, a sheet feeding device for an image forming apparatus includes a plurality of separating members having the same coefficient of friction with respect to sheets for separating the sheets one by one, an automatic separating member switching mechanism including a drive source for automatically replacing the separating members, a counter for counting the sheets fed, and a controller for so controlling, when the counter counts a preselected number of sheets, the drive source as to automatically replace one separating member in use with another separating member.